Originally posted by LtNarol
Black holes are supposed to be small...they're also not supposed to be visible since light can't escape them, but thats ok 
Nice graphics by the way
Bad Narol! Back to elementary cosmology for you!
The accretion disc is visible from the outer edge to the event horizon because the matter is swirling in, constantly accellerating. As it gets pulled closer in and moves faster, there's less room for it to move, and collisions start happening. Pretty soon, you're seeing energy being bled off as light.
Singularities are small, but the black hole (as defined by the event horizon can be quite large, because it is defined by the mass of the singularity. As Venom said, the SMBH at the center of the Milky Way is absolutely massive.